Aqueous dispersions of ethylene-acid copolymers and ionomers are in many applications preferred over the use of the ethylene-copolymer per se as they provide several advantages. The advantages include: low process requirements, as dispersions do not require the use of heavy and expensive processing equipment, such as for example extruders; application of a thin coating film as coatings of only a few micrometers can be applied instead of a minimum thickness limitation of several tens of micrometers present in the melt polymer application; as well as an environmentally friendly system, as these dispersions are water-based systems.
Furthermore these dispersions usually are dried after application accompanied by the evaporation of water and therefore they possess the properties typical of ethylene-acid copolymers or ionomers. Dispersions have excellent filming and sealing properties that can be used for applications such as coatings or adhesives on foil, metal, paper, polymer or textiles. They also show cross-linking capability and excellent adhesion for modifiers or additives in paints or inks; binders for non-wovens; rustproof aqueous coating or antistatic coating material. The dispersions can be applied by many different processes including immersion, spraying and processes using a coater device (such as an air knife, blade, gravure roll or metering rod coater), depending on the substrate and the effect desired on the application.
Ethylene-acid copolymers, such as resins available from DuPont under the Trademark NUCREL®, can be dispersed in water under appropriate conditions. These dispersions can be produced in water via an organic medium. For example, U.S. Pat. No. 4,351,931 describes low-acid low-MFI copolymers obtainable as dispersions by emulsifying a polymer solution in a non-miscible solvent in water, and removing the organic solvent. Another method for producing aqueous dispersions of ethylene-acid copolymers is by mixing the resin pellets and water with a base under agitation and at temperatures above the melting point of the resin. This second method is more preferable in producing ethylene-acid copolymer aqueous dispersions, as it is environmentally friendly (no organic solvent present during dispersion production) and it is a one-step production process. The invention discussed herein and the following discussion refers to the second method of dispersion production described above.
During dispersion formation the base cation reacts with the copolymer acid groups thus neutralizing the acid copolymer and converting the copolymer into an ionomer. This reaction induces high polarity in the copolymer that provides high stability of the dispersion and therefore no surfactants or other additives are required. Ethylene-copolymer ionomers, such as resins available from DuPont under the Trademark SURLYN®, can be dispersed in water, with no need for a base or any other additive, under agitation and at temperatures above the melting point of the resin.
The ethylene-copolymers that are the easiest to disperse are those that have high MFI and high acid content. The high MFI (or correspondingly low molecular weight) provides for easier solubility of the copolymer while the high acid content provides for higher hydrophilicity of the copolymer, both of which result in better dispersibility. Typical commercial dispersible resins have MFI higher than 300 and an acid content of 20 wt. %. As the MFI and/or the acid content decrease, the resin becomes more difficult to disperse as exhibited by an increase in the amount of non-dispersible material.
However, low MFI and/or low acid content are desirable in many dispersion applications. This is due to the fact that low MFI provides for properties such as improved scratch and abrasion resistance, and hot tack (seal strength), while low acid content provides for better adhesion to polymer substrates such as LDPE (low density polyethylene). Both properties provide better chemical resistance to the coating.
The dispersibility of an ethylene copolymer can be affected by the control of the following key factors, namely: (1) the type of base used, (2) the amount of base used, (3) the type of acid in the copolymer and (4) the process conditions during dispersion production, as follows:
(1) Type of base: Typical bases used for dispersions can be divided into two major categories. The first includes metal-based bases, such as those based on alkali and alkaline earth cations. Examples of these are sodium hydroxide (NaOH), potassium hydroxide (KOH), or zinc oxide (ZnO). The second category includes volatile bases including ammonium hydroxide (NH4OH) or amines. The type of base selected is very critical for the dispersion application. Bases belonging in the first category provide the dispersion with high performance properties typical of ionomer copolymers. These include higher cross-linking, higher chemical resistance and improved mechanical properties. Bases belonging in the second category are vaporized after dispersion application and therefore are more preferred in applications where the presence of metals is not desirable and for providing higher water resistance of the dispersion coating.
(2) Amount of base: The amount of base added during dispersion production determines the level of neutralization of the copolymer, as it relates to the number of acid groups that have been neutralized by the base cation. Therefore the degree of neutralization determines the polarity of the copolymer, and the higher the neutralization level the better the dispersibility. It is observed that with increasing neutralization the particle size of the dispersion decreases but the viscosity of the dispersion increases significantly. Lower neutralization can provide for higher solids content dispersions as well as retaining an amount of carboxylic acid groups intact which can be desirable for many applications.
(3) Type of acid: The most common acids used for ethylene-copolymer dispersible resins are acrylic acid and methacrylic acid. Although both are carboxylic acids and differ only by one methyl group they exhibit very different behavior on the dispersibility of the resin. This is mostly pronounced in dispersions based on ammonium-type bases compared to alkali-based dispersions. In this case it is observed that for a typical acid content of 20 wt. % and MFI of 300, ethylene-acrylic acid copolymers can be dispersed while ethylene-methacrylic acid copolymers cannot. This provides significant limitations in the use of ethylene-methacrylic acid copolymers in a broad range of dispersion applications. On the other hand both copolymers are dispersable in alkali bases, such as NaOH.
(4) Dispersion process conditions: The basic process conditions for producing a dispersion include: temperature, pressure and agitation speed. In general, a dispersion is produced at temperatures above the resin melting point. The higher the process temperature the easier the resin disperses. Typical process temperatures are 100° C.-150° C. However, if the dispersion process temperature is set above about 100° C. (the normal boiling pressure of water) then pressure needs to be applied. Agitation enables the breakage of the resin pellets into the formation of dispersion particles. Thus, higher process temperatures and higher agitation speeds are needed for easier dispersion formation.
U.S. Pat. No. 5,387,635 proposes a method to produce dispersions of low MFI and/or low acid content ethylene-acid copolymers with low non-dispersible material according to which successful dispersions based on these resins can be produced only using appropriate proportions of mixtures of bases comprising ammonium hydroxide and alkali metal hydroxides. The Examples of U.S. Pat. No. 5,387,635 show that ethylene-acrylic acid copolymers of low MFI and/or low acid content cannot be dispersed in single bases, such as NaOH, KOH or NH4H, as this results in a large amount of nondispersible material, and the patent instead teaches the use of mixed bases at given concentrations.
However, use of mixed bases in a dispersion can be undesirable in many of the dispersion applications that require for example the presence of non-metal in the dispersion, or that require high water resistance of the dispersion coating.
PCT/US94/04654 describes a base copolymer film coated with a composition comprising a copolymer of ethylene and acrylic or methacrylic acid, wherein 2 to 80% by weight of the carboxylate groups are neutralized with metal ions from groups Ia, IIa and IIb of the Periodic Table. The coating according to this document is suitable as a heat sealable coating for fast runing packaging machines. A crucial factor is the combination of hot tack and a minimum seal temperature. The neutralization with metal ions serves to improve hot tack. However, the drawback of the neutralization with metal ions is that it has a negative influence on the minimum seal temperature which is an important property in packaging films.
U.S. Pat. No. 4,173,669 describes an aqueous dispersion for coating comprising a partially neutralized copolymer comprising 99 to 75 mol % ethylene and 1 to 25 mol % of α,β-ethylenically unsaturated carboxylic acid, of which at least 10% is neutralized with metallic and/or ammonium ions, the copolymer having a specific distribution of acidic units. These copolymers are dispersed at relatively high temperature, 120° C. or higher. In the examples only ethylene methacrylic acid copolymers are produced which are neutralized with either sodium or a combination of sodium and ammonium ions. With the described process it is not possible to produce ethylene acrylic acid copolymers dispersible in aqueous ammonium hydroxide at low temperatures.
Dispersions based on low MFI resins are preferred in many of the dispersion applications as they have improved properties compared to higher MFI resin dispersions, but to date it has not been possible to disperse low MFI resins in a problem-free manner, unless high dispersion process conditions (high temperature or pressure) are applied. A further preferred property in the dispersions is that they are neutralized with ammonium ions only, to provide low minimum heat seal temperatures.